The invention relates to a method and apparatus for dielectric heating by means of microwave energy.
It is known that, in order to achieve a high degree of efficiency for a microwave heater, the load must be matched to the microwave generator and its waveguide, as the microwave generator, the energy waveguide, the treatment chamber and the load may have different impedances. If the impedances of the microwave generator, the waveguide and the treatment chamber on the one hand and the impedance of the load on the other hand are different, transformation means must be interposed between them, the purpose of the transformation means being to match the impedance of the load to the impedance of the microwave generator with the wave guide and the treatment chamber. Optimum utilization of the microwave energy is ensured only when the entire quantity of energy produced in the microwave generator is converted to heat in the load. This means that as far as possible, there should not be any reflection of the microwave energy at the load.
In order to provide for low-reflection matching of the load, it has been proposed to provide one or more tuning pins or tuning screws to project displaceably into the waveguide, which waveguide may be in the form of a hollow waveguide passage. With such tuning members which act as a matching transformer, it is theoretically possible for any load impedance to be matched to any generator impedance.
It has been found that the impedance of the load is variable, in operation of a microwave heater. Therefore, it is not sufficient for the load to be matched just once to the microwave generator by way of the tuning members. There is therefore a requirement for automatic matching of the microwave heater during operation thereof. This automatic matching operation is intended to reduce to a minimum the amount of microwave energy which is reflected by the load, as a result of defective matching.
It has already been proposed that the microwave energy reflected by the load may be measured and the tuning members adjusted in accordance with the measured value, in order to reduce to a minimum the microwave energy which is reflected by the load. However, it is not possible to give a clear adjustment instruction, over a large range of variation of the reflection coefficient.